For some time research has been conducted on a chemometrics-based optical measurement method called multivariate optical computing (MOC). A phenomenon in MOC has been identified and designated as the MOC passband disadvantage that has been defined as the cost of including large spectral windows in which the sample shows no absorbance. The phenomenon is analogous to the multiplex disadvantage sometimes observed in FT-Raman spectroscopy of weak bands in the presence of stronger features. The passband disadvantage increases noise in a measurement without improving, and sometimes harming, the ability to chemometrically model a chemical system.
The MOC passband disadvantage, like the FT Raman multiplex disadvantage, is addressed by restricting the spectral band of a measuring device to wavelengths of greatest interest using a physical optics means like filtering or by using special light sources, etc. However, many of the best and most convenient methods for physical wavelength selection have undesirable consequences such as irreproducibility. Interference filters, for instance, vary from production lot to production lot, and can even vary within a single lot. For this reason, the physical properties of bulk materials have generally been relied on to provide the most stable wavelength selection. This same phenomenon affects the reproducibility of simple bandpass photometers, where the filtering elements vary from instrument to instrument, making calibrations instrument-dependent.
A more nearly ideal selection of wavelengths would be made by using detectors whose wavelength response is tuned more directly and reproducibly toward the spectral intensity of the analyte whose measurement is sought. If such a detector could be created and if it were convenient to use, then its responses would better correlate with analyte concentration in mixtures than would those of a broadband detector, even in the absence of any additional treatment. This would improve the consistency of photometers and, if used in a MOC system, one might expect such a detector to provide enhanced performance and reduced sensitivity to spectral interferences.
A direct approach to creating a simple detector with a spectral response tuned or adjusted to the absorption bands of an analyte is to base it on thermal detection methods. Photothermal and photoacoustic methods provide signals proportional to energy loss following absorption by detecting the conversion of light into heat by non-radiative decay. A detector in which the pure analyte or mixture of analytes is used as the detection medium in a photothermal or photoacoustic measurement can serve to restrict the wavelength band. The same concept has been previously demonstrated using Golay cells.
Solid-state detectors based on the detection of evolved heat are more commonly used in optical systems. Pyroelectric detectors, thermocouple or thermopile detectors, bolometers, etc. are all relatively sensitive, broadband detectors based on the detection of heat or temperature. In each case, the detection of light over a wide range of wavelengths is accomplished by converting it to heat and measuring a temperature-dependent detector property. Compared to photon detectors, thermal detectors usually have lower detectivities (D*), but a wider and more featureless spectral response. For a photodector, detectivity, D*, is generally defined as a figure of merit used to characterize performance, and is equal to the reciprocal of noise equivalent power (NEP), normalized to unit area and unit bandwidth.
While various implementations of temperature-dependent detectors have been developed, no design has emerged that generally encompasses all of the desired characteristics as hereafter presented in accordance with the subject technology.